Theoretical Study About Formaldehyde Adsorption On The Surface Of Kaolinite And Kaolinite Doped Metal And Nonmetal

As the main indoor harmful gas compositions,formaldehyde threats human health seriously.The effective method to eliminate formaldehyde has been the focus of researchers.Kaolinite resource is abundant in our country.Because of its stable structural properties and low-priced,it is widely used as adsorbent to eliminate air pollution.Especially the modified kaolinite has wide application prospects in the elimination of organic small molecules,heavy metal ions and other harmful gases.It is also of great practical significance to find a modified kaolin with effective adsorption of formaldehyde.In this work,all quantum calculations based on the Generalized gradient Approximation(GGA)functional of density functional theory were performed at DMol3 module in the Material Studio package.The geometries and electron structures of formaldehyde adsorbed on pure kaolinite(001)surface and kaolinite(001)surface doped metal and nonmetal were studied,in which the change of adsorption energy,bond length,charge density map,total density of States,atomic density of States and Mulliken charge population were compared in the paper.In the study of formaldehyde adsorbed on pure kaolinite(001)surface system,the geometries were optimized,the energy of the geometries were calculated.There were 6 types of-OH in each unit of pure kaolinite(001)adsorbed and three positions to be able absorb formaldehyde in every types of–OH.So 18 kinds of stable adsorption structures were calculated and compared.By the comparison with absorption energies,all the stable structures are physical adsorption,and the structure of adsorption of formaldehyde on the No.1-OH is more stable,and the structure of HK1801 is most stable,its absorption energies is 49.8 kJ/mol.Although the bond length C=O in the most stable structure was stretched comparing to formaldehyde,the electron cloud did not overlap.Therefore the interaction was strong physical adsorption.The structure of formaldehyde adsorbed on kaolinite(001)surface single doped metal(Fe,Co,Ni)were optimized.The changes of the structures,the total density of states,the corresponding atomic density of States and Mulliken charge population were compared with after and before the doping.It is found that,comparing with the pure kaolin(001)surface,there was a little change for the energy of Fe doped system,and absorption energy was just49.4 k J/mol.Due to the appearance of impurity peaks and the increasing of valence band bandwidth,the band gap is obviously narrowed,which is beneficial to formaldehyde C=O bond activation.The performance of Co doped system is slightly worse,and the absorption energy for best absorbed structure is 31.4kJ/mol,in which the conduction band just moves down and shows weak physical adsorption.The absorption energy of system doped Ni was 65.9 kJ/mol,and the d electrons enters into top of the valence band.The formation of impurity level in the top of valence band lead to band gap narrowed.Formaldehyde adsorption is strengthened,and the adsorption type is strong physical adsorption.The behavior of formaldehyde adsorbed on kaolinite(001)surface singledoped non-metallic(S,N)was studied in detail.The characters of the adsorption energy,charge density,total density of states,atomic partial densities of states,and Mulliken charge population were investigated.The adsorption energy of the system doped S and N were 41.3 and 33.5kJ/mol,respectively.The C=O bond elongation of formaldehyde was 0.005? in the best adsorption mode,and the adsorption effect was less than that of pure kaolinite.The doping of S and N inhibited the adsorption of formaldehyde and it still belonged to weak physical adsorption.